The Pachaug River is a 16.0-mile-long (25.7 km) river arising from the Pachaug State Forest at the Connecticut - Rhode Island border and draining into the Quinebaug River. It is crossed by the Ashland Mill Bridge in Griswold, Connecticut, a bridge which is listed on the U.S. National Register of Historic Places.Hopeville Pond

Pachaug State Forest is the largest forest in the Connecticut state forest system, encompassing over 27,000 acres (110 km²) of land. It is located on the Rhode Island border in New London County, and parcels of the forest lie in the towns of Voluntown, Griswold, Plainfield, Sterling, North Stonington, and Preston. The forest was founded in 1928, but most of the land came from purchases made later during the Great Depression. It is named after the Pachaug River, which runs through the center of the forest. The forest is part of the Northeastern coastal forests ecoregion.